To reach optimum returns on your emails, invest some time in filtering your email strategy through your brand standards and your marketing goals to arrive at a selection of possible messaging approaches. Then test the variables, using a basic A/B split or any other segmentation that is important you.

Here are a few things you can test:

1. OfferWhat offer will move your audience closer to a purchase? You may intuit the answer, or have to follow a business directive to make a particular offer, but by all means test themes and variations to create an offer the audience can't refuse. (Be careful of too much product discounting unless you want your brand to be associated with "cheap.")

Financial incentives: money off, free shipping, two for the price of one, discounted service contract or installation.

Time-based incentives: 3-day sale, offer ends Saturday, for the month of May only.

2. Audience empathyTest ways of connecting on an emotional or intellectual level with your prospects.

Exclusivity: be the first to own, leave the crowds behind, apply for exclusive entry.

Problem-solving: a better way to ___, take the back-break out of ___, get results faster, smarter, more reliably.

Emotion: because you love your children / pet / garden / beer, because you care about this issue / institution / group.

3. Tone and mannerWhile these must follow your brand standards, email is inherently a more personal medium than many other channels, so you can test some variables here.

Timbre: taking a warm, personal approach versus a more clinical exposition of facts.

Length of copy: test long, medium and short copy (depending on the product or service).

Persuasion versus urgency: test to see if some of your audience wants to be schmoozed with reasons to buy (product attributes and excellence) versus receiving nonstop promotions.

4. StructureWhere the offer appears may make a difference in how it is understood and acted on.

Placement: try placing offers in the subject line, the headline, the call to action, a sidebar, and in image captions and call-outs. Make each instance a link and then track opens and where users click.

Graphics -- be careful with presenting headlines and key offers in graphics, which may well get blocked from view in the inbox. Simpler is often better.

You can test which copy approach yields the best results simply by tracking click-throughs and conversions on sent emails. To dive deeper, you can also perform other types of research. Use focus groups or surveys to test the impact and takeaway of your offers and the way they are worded. You can also do eye-tracking studies to find out how readers scan your emails. When you learn what path their eyes follow, you can maximize the effect by placing your key inducements, either copy points or visuals, along the same path, in order of importance.

There are almost always several good ways of writing and designing any advertising message. If you try to test every possible combination of variables, you'll go cross-eyed. Apply lessons learned from your marketing in other channels to your email advertising, and vice-versa, and you will find the options narrowing nicely into a powerful set of guidelines you can use creatively again and again.